temporarily store identifiers for delayed matching somewhere other than the DB

[kelseyq]

There is a settings option (MATCH_IMMEDIATELY=False) which allows you to schedule the check for new matches to be run on a regular basis rather than running it immediately when a new match is submitted. Since #68, when this setting is set to False, we store the perpetrator identifier in the DB alongside the encrypted report so it can be used in the matching check when it ends up running, and then delete the identifier after it's been used in matching. This could end up with the identifier being preserved in automated DB backups, which could be problematic in the case of subpoenas. We only need to store the identifiers themselves--they don't need to be associated with a MatchRecord. We could store them in a temporary file that we overwrite, which might be less persistent.